I'd say one of the major problems involved with this is players being able to use Auto-Reraise to their advantage by using 'zombie' tactics. Although it is possible to do now, outside of Abyssea this still has the restriction of recasting a Reraise spell to hold claim or delay a monster to either move it to a different location, or wait on arriving players or players healing from weakness.

And also, if this trait was given to WHM, why would they not be in the habit of casting Reraise III on themselves anyway? MP concerns aren't so much an issue anymore with either Atmas inside Abyssea or Refresh II/Ballad III and more 'Refresh+' gear outside Abyssea.

Although Auto-Reraise works in previous FF titles, I think that this trait is more suited towards the Single-Player titles to avoid the 'Game Over' screen and isn't really needed in an MMO.